We are engaged in the development of a general, quantitative theory of response evocation based on the principle that the decision criterion is a random variable. The intent is to identify ad describe the theoretical processes which intervene between stimulation and response evocation. The experimental procedures involve simple, disjunctive, and choice reaction time. Research problems under investigation include variables related to stimulus and response expectancy, visual search, and item recognition. Considerable emphasis is placed on attempts to describe, at a theoretical level, individual differences in information processing dynamics.